PyHPC: Rewrite mpi4py lesson with heat stencil.

[robobryce]

Summary

• Rewrite the mpi4py notebook as a concept-to-validation lesson with one simple distributed reduction exercise and one advanced 2D heat-equation stencil exercise.
• Add a paired solution notebook with executed outputs and serial-reference validation.
• Select a compatible MPICH/CSCS or Open MPI launcher and propagate child-process failures to notebook execution.
• Update PyHPC notebook links, test documentation, and generated-artifact ignores.
• Use no profiling tools.

Testing

• python3 brev/test-notebook-format.py tutorials/pyhpc/notebooks/03__mpi4py.ipynb tutorials/pyhpc/notebooks/solutions/03__mpi4py__SOLUTION.ipynb tutorials/pyhpc/notebooks/syllabi/pyhpc__cupy_mpi4py_jax_cppinterop_kernels__8_hours.ipynb
• pytest tutorials/pyhpc/test/test_notebooks.py -k 03__mpi4py using an MPICH-built mpi4py environment

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